This invention relates to a system for monitoring the air/fuel ratio in an internal combustion engine by using an oxygen sensor of the concentration cell type disposed in the exhaust gas.
In recent automotive internal combustion engines it is common to control the air/fuel mixing ratio precisely to a predetermined optimum value by performing feedback control. In many cases the target value of the air/fuel ratio is a stoichiometric air/fuel ratio. For example, when a so-called three-way catalyst is used in the exhaust system to achieve simultaneous reduction of NO.sub.x and oxidation of CO and HC, the air/fuel ratio must be controlled precisely to the stoichiometric ratio because this catalyst exhibits best conversion efficiencies in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture. In the current feedback control systems, it is therefore usual to produce a feedback signal by sensing changes in the concentration of oxygen in the exhaust gas.
The sensing device which measures concentration in the exhaust gas, thereby monitoring the air/fuel ratio in the engine, usually employs an oxygen sensor of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte, such as zirconia, stabilized by calcia or yttria, and two electrode layers formed on the outer and inner surfaces of the solid electrolyte layer, respectively. An oxygen sensor of this category, suitable for use in a feedback control system which aims at the stoichiometric air/fuel ratio is produced by making both the solid electrolyte layer and the outer electrode layer permeable to gas molecules. When this oxygen sensor is disposed in the exhaust passage of an internal combustion engine with the outer electrode layer exposed to the exhaust gas, an oxygen partial pressure in the exhaust gas always acts on the outer electrode layer. Furthermore, an oxygen partial pressure is produced at the inner electrode layer by reason of inward diffusion of oxygen contained in the exhaust gas through the microscopically porous solid electrolyte layer. However, the oxygen partial pressure at the inner electrode layer does not instantaneously follow a change in the oxygen partial pressure in the exhaust gas since the solid electrolyte layer is relatively low in permeability and offers some resistance to the diffusion of oxygen molecules therethrough. Therefore, when a considerable change is produced in the concentration of oxygen in the exhaust gas by a change in the air/fuel ratio in the engine across the stoichiometric ratio, a great difference arises between the oxygen partial pressure at the outer electrode layer and that at the inner electrode layer. This causes the output voltage of the oxygen sensor to exhibit a sharp change from a high level to a low level, or vice versa. Such a change in the output voltage of the oxygen sensor can easily be detected by continuously comparing the sensor output voltage with a suitably predetermined reference voltage.
However, under some conditions the accuracy of the air/fuel ratio monitoring by the above described method is not reliable. For example, during operation of the engine under transitional conditions there is the possibility of a considerable rise or fall in an average level of the output voltage of the oxygen sensor, whereas the aforementioned reference voltage remains unchanged. This leads to the possibility that the output voltage of the oxygen sensor does not intersect the reference voltage even though the actual air/fuel ratio changes across the stoichiometric ratio, so that the air/fuel ratio is misjudged. Furthermore, a change in an average level of the oxygen sensor output voltage is probable as the oxygen sensor is used for a long time.
To solve the above described problem, Japanese patent application primary publication No. 58-144649 and corresponding British patent application publication No. 2,115,158A propose an air/fuel ratio monitoring system, in which the reference voltage with which the output of the oxygen sensor is compared is made variable depending on the level of the oxygen sensor output voltage. That is, the reference voltage is produced by first producing a variable voltage signal. The variable voltage signal is obtained by adding or subtracting a fixed voltage value to the oxygen sensor output when the sensor output indicates that the air/fuel ratio is above or below the stoichiometric ratio, respectively. The variable voltage signal is smoothed in an RC circuit to a variable reference voltage. The time constant of the RC circuit is set at a fairly large value so that, when the oxygen sensor output voltage steeply varies in response to a change in the air/fuel ratio across the stoichiometric ratio, the reference voltage varies at a lower rate than the sensor output voltage to ensure that the varying sensor output voltage intersects the reference voltage. This air/fuel ratio monitoring system is certainly improved in accuracy. However, when the attenuation of the sensor output voltage due to a gradual change in the oxygen partial pressure at the inner electrode of the oxygen sensor takes place at a relatively high rate, the attenuating sensor output voltage may intersect the reference voltage which is varying at a relatively low rate. Then, the system will incorrectly indicate that the air/fuel ratio has crossed the stoichiometric ratio.